In Japan, the NOx storage reduction (NSR) catalyst also known as NOx trap or NOx adsorbent is a demonstrated after treatment technology for control of HC, CO, and NOx on vehicles equipped with lean burn gasoline engines. This catalyst provides two key functions. When the engine operates with a stoichiometric air/fuel ratio, it functions as a standard three-way conversion catalyst. Under lean operating conditions, while CO and HC in the exhaust are combusted, the NSR catalyst trap functions as a trap for NOx (NO+NO2). The reaction mechanism of NOx storage and reduction over a NSR catalyst trap are depicted in Equations 1-4. In general, a NSR catalyst trap should exhibit both oxidation and reduction functions. In a lean environment, NO is oxidized to NO2 (Equation 1). This reaction is catalyzed by a noble metal (e.g., Pt). Further oxidation of NO2 to nitrate, with incorporation of an atomic oxygen occurs. The nitrate is then stored over selected metal components (Equation 2). To ensure continuous and lasting NOx control, the NSR catalyst trap requires periodic regeneration with controlled short, rich pulses, which serve to release (Equation 3) and reduce the stored NOx (Equation 4). Again a Pt group metal is used for NOx release and reduction. Poisoning of the NSR catalyst trap by sulfur oxides takes place in principle in the same way as the storage of nitrogen oxides. The sulfur dioxide emitted by the engine is oxidized to sulfur trioxide on the catalytically active noble metal component (e.g., Pt) of the NSR catalyst trap (Equation 5). Sulfur trioxide (SO3) reacts with the storage materials (e.g., Ba) in the NSR catalyst trap with the formation of the corresponding sulfates (Equation 6). Because of the low capacity of the trap to hold sulfur before activity falls and of the stability of sulfate poisons, frequent high temperature desulfations under fuel rich conditions are required (>650° C.). This stresses the thermal stability of the NSR catalyst trap and ultimately results in a significant fuel penalty as a result of running a fuel rich mixture as required for high temperature desulfations. This correspondingly shortens NSR catalyst trap life.
Equations:
(1)NO + 1/2 O2 = NO2Oxidation of NO to NO2(2)2NO2 + MCO3 + 1/2 O2 = M(NO3)2 +NOx Storage as NitrateCO2(3)M(NO3)2 + 2CO = MCO3 + NO2 +NOx release:NO + CO2(4)NO + NO2 + 3CO = N2 + 3CO2NOx reduction to N2(5)SO2 + 1/2 O2= SO3SOx poisoning Process(6)SO3 + MCO3 = MSO4 + CO2SOx poisoning Process
In equations 2, 3 and 6, M represents a divalent base metal cation (e.g., Ba). M can also be a monovalent or trivalent metal compound, in which case the equations need to be rebalanced.
One method for decreasing the formation of sulfates that poison the NSR catalyst trap is to provide a SOx trap upstream of the NSR catalyst trap which undergoes a continuous sulfur uptake and release as a function of the air/fuel ratio (A/F ratio). By periodically changing the exhaust gas conditions from lean to rich, the sulfates stored on sulfur trap are decomposed to yield sulfur species, and the nitrates stored on the NSR catalyst trap are reduced to nitrogen. Key requirements are that a substantial fraction of sulfur species released pass through the NSR catalyst trap with no poisoning of the NOx storage (e.g., Ba) and reduction components (e.g., Pt).
EP 0582917 A1 discloses that the poisoning of a storage catalyst with sulfur can be reduced by a sulfur trap inserted into the exhaust gas stream upstream of the storage catalyst. Alkali metals (potassium, sodium, lithium and cesium), alkaline earth metals (barium and calcium), and rare earth metals (lanthanum and yttrium) are disclosed as storage materials for the sulfur trap. The sulfur trap also includes platinum (Pt) as a catalytically active component. However the disadvantage of the embodiments in EP 0582917 A1 is that the sulfur storage capacity is limited, unless an inordinately large trap is provided or the trap is replaced at very frequent intervals. Once the sulfur trap reaches its full storage capacity sulfur oxides contained in the exhaust gas will pass through the sulfur trap and poison the NSR catalyst trap.
U.S. Pat. No. 5,473,890 discloses a SOx trap composition selected from alkali, alkali-earth, and rare earth metals. Pt is also added to this formulation. High temperature regeneration (>650° C.) is needed for such a system, which is not a practical solution since this will result in thermal damage to this trap and the NSR unit in the same flow line. U.S. Pat. No. 5,473,890 refers to a SOx trap containing at least one member selected from copper, iron, manganese, nickel sodium, titanium, lithium and titania. In addition Pt is added to the catalyst. Pt containing adsorbents result in significant quantities of H2S release under rich conditions, which will react with sulfur trap components forming stable metal sulfide leading to only a partial regeneration of SOx trap. The authors did not show any test activity for the system.
U.S. Pat. No. 5,687,565 discloses a very complex oxide composition, selected from alkaline earth oxides (Mg, Ca, Sr, Ba, Zn). In addition Cu and noble metals (Pt, Pd, Ru) were also added. Again such a system is unpractical as a regenerable SOx trap due to the need for 650° C.+ regeneration and the poisoning effects of H2S release.
U.S. Pat. No. 5,792,436 discloses SOx traps containing alkaline earth metal oxides selected from Mg, Ca, Sr, Ba in combination with oxides of cerium and a group of elements of atomic numbers from 22 to 29. Pt is also added to the catalysts formulation. Again such a system requires high temperatures to regenerate (>650° C.).
EP 1374978 A1 discloses SOx traps containing oxides of copper. The authors indicate that the system can be regenerated at low temperature (250-400° C.) depending on the support. However, the authors did not show any data on the effect of the released sulfur species (e.g., SO2) on NSR catalyst trap. As will be discussed later, the released SO2 at these low temperatures will poison NSR reduction sites under rich conditions.
U.S. Pat. No. 6,145,303 discloses H2S formation under rich conditions, and a method to suppress it when the air/fuel ratio is close to stoichiometry. This approach to suppress H2S formation translates into a partial and a long regeneration period of the sulfur trap. Moreover, a higher temperature is needed for desulfation, which can also stress the thermal stability of the sulfur trap.
WO 0156686 discloses that the release of sulfur under rich conditions leads to the adsorption of sulfur species on NSR. Also disclosed is that such sulfur adsorption will affect the NSR catalyst trap and a high temperature desulfation procedure of the NSR catalyst trap is needed.
The aforementioned methods for operating an exhaust gas treatment unit consisting of a sulfur trap and a nitrogen oxides storage reduction catalyst have two distinct disadvantages. The first disadvantage is the absence of a procedure to transmit sulfur species through NSR catalyst trap with no poisoning of NOx storage and reduction sites. The second disadvantage is that most of the reported sulfur traps contain Pt and are partially regenerated at high temperatures releasing H2S as main product. In addition H2S may be an issue for future regulation and need to be controlled.
A need exists for an improved process for operating an exhaust gas treatment unit including a sulfur trap and a NSR catalyst trap operated in tandem. The system will ideally have a SOx trap regenerable at moderate temperatures (˜400-600° C.) by use of a regeneration gas media that can enable the sulfur species released from sulfur trap to pass through the NSR catalyst trap with no poisoning of NOx storage and catalytic components.